1. Field of the Invention
The invention concerns a process for laser machining a coated sheet metal and a coated sheet metal. A process and a sheet metal of this generic type are already known from DE 44 07 190 A1.
In the case of many coated sheets, in particular galvanized or zinc coated sheet metal as employed in the automobile industry, the coating material exhibits a significantly lower melting point than the melting point of the sheet material. Accordingly, during laser welding of this type of sheet, explosive-like vaporization of coating material can occur in an overlap joint, which strongly compromises the quality of the joint.
For improving the joint quality it has already been proposed to use spacers to produce a narrow gap between the sheets, so that the vaporized coating material can escape. Suitable crater-shaped spacers can be produced according to JP 11-047967 by laser radiation of the surface. According to DE 44 07 190 A1, nub-like spacers can be produced using a pulsed laser beam. The possibility of influencing the geometry of the spacers is not disclosed.
The primary disadvantage therein is that the spacers produced in this manner are relatively pointy. As a result, they relatively easily penetrate—depending upon the applied forces—and enter the adjacent sheet metal, or themselves become deformed, whereby undesired deviation in sheet spacing occurs. In the case of thin sheet metal, the nubs can cause raised impressions on the opposite side of the sheet metal being spaced.
The task of the present invention is to reduce the deviations in the sheet spacing by suitably shaping the spacers.
2. Description of the Related Art
The invention, with regard to the process to be provided, is inventively solved thereby, that on a least one side of at least one coated sheet metal at least one topographic change projecting from the surface is produced by means of a laser, wherein the laser beam produces the at least one topographic change, by carrying out a movement with transverse and longitudinal components through and/or about the center of a processing surface.
The advantage of this design is comprised therein, that a movement of the laser beam occurs within the interaction zone of the melt, so that in addition to the mixing induced by the heating, it excites or quasi stirs the melt. This leads thereto, that the resulting topographic change becomes more “spherical” shaped at it's apex, so that it exhibits an apex radius that is greater than the height of the topographic change. This type of topographic change is more suitable as a spacer than those previously known, since, due to its spheroid shape, it is less likely to press into the opposite sheet metal or itself become deformed, and thus less undesired deviations in sheet spacing would occur. Besides this, even in the case that the sheet metal is thin, no imprints are created by the topographic changes on the opposite side of the sheet metal being spaced. Further, coated sheets with the topographic changes produced in the inventive manner exhibit an improved corrosion resistance compared to those produced by previously known methods. On the one hand, a spherical peak is less likely to penetrate into the sheet metal to be spaced apart and thus causes little or no damage to the coating. On the other hand, the depression of the sheet metal, from which material of the projecting topographic changes are formed, is fundamentally flatter in shape than those produced by known methods, and thus has less of a tendency to harbor moisture (less capillary effect).
It is particularly preferred when the laser beam is guided on the surface by means of a scanner device. The scanner device is a particularly rapid and flexible beam deflection device, for example a mirror system (comprising at least one single- or multi-access controllable pivotable mirror) or also acousto-optic modulators.